Raft Foundation

[Karol2010]

Hello

Can anyone help me. I want to use a steped toe raft foundation to build a 2 storey blockwork house on. The outer walls are 2no. 100mm blocks with 100 cavity between. Hollowcore 200mm to first floor. Traditional cut timber roof. I figure the load to be 60KN/m run. The ground bearing capacity at 90Kn/msq. I do not have any software to do a design so if anyone could detail the minimun required steel in the toe that would be great. Also there will be 400mm compacted stone to the underside of the raft throughout.

 

[ishvaaag]

Since you say raft ... must I interpret you want use a mat foundation on the whole plan? Or just a footing running under the perimetral wall? A footing like this with a section like a podium? I also interpret all the building over the ground, no cellar.

Then for this case a proper analysis first would determine the loads on the foundation and then ensure the one provided meets the requirements. A model including both would be even better but likely overkilling for a 2 stories work. Design of these things varies with the view of the undersigning party; I had a colleague that wanted this kind of foundations with just a bottom layer reinforcement, there are people with more will than refined judgement.

Hence you need to determine your solicitations. The most basic is the reinforcement in cantilever as a pair of transverse cantilevers under the bearing wall. The pressure on the soil must equilibrate what applied, factored. Then a qL^2/2 is what you have and usually the depth of the taller part of the stepped section as structural depth. Need to check shear. Then for longitudinal direction either you have performed some kind of analysis that gives you solicitations or you are entirely lost to that respect. For any solicitations, the code will mandate typically minimum ratios of reinforcement based in both the section and the solicitation. You need to look the applicable code where such minimums should be quite plainly stated. But even if you comply with them you might not have a structurally sound foundation without an analysis giving the solicitations in the beam. Such analysis might reveal or at least give hint about that for a soft soil like yours you could be getting differential settlement incompatible with a sound behaviour of the masonry walls above.

 

[ishvaaag]

When I say factored, of course, for determination of the reinforcement.

 

[BAretired]

I do not have any software to do a design so if anyone could detail the minimun required steel in the toe that would be great. Also there will be 400mm compacted stone to the underside of the raft throughout.

Get real! Who would assume responsibility for that design? If you can't design it yourself, find someone who can.

BA

 

[apsix]

What's that BA, are offering to draft it as well?